Allen, as you mentioned, you ARE using this primer in the Hornet case. But, the Rem 7-1/2 primer is too hot a primer for the Hornet case. You'll find in using the 7-1/2, initial primer ignition will indeed start the bullet moving in the case prior to full powder burning. Being it is about the hottest primer we have access to, it is not suitable for the small Hornet case for this reason alone.
You'll most likely find the best compromise using something milder like the WSR. I use the 6-1/2 in my 22 Hornets and K-Hornets, along with my 17AH with perfect results, but many 17AH's seem to like other mild primers such as the WSR. There's been alot of experimentation on this, and hot primers will move a bullet in the Hornet case prematurely and cause irratic accuracy and velocity readings. This is why many use pistol primers in the Hornet case. Keep in mind also that the 17AH and the 22 Hornet operate at different operating pressures, but both are subject to the weak Hornet case design as well as the small internal capacity of the Hornet case makes it very suceptable to primer intensity.
Check out some primer ratings here:
http://www.jamescalhoon.com/primers_and_pressure.php
Another good read on the subject:
http://www.sksboards.com/smf/index.php?topic=56422.0
